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# Re: st: power calculation graphs

 From Nikolaos Pandis To statalist@hsphsun2.harvard.edu Subject Re: st: power calculation graphs Date Fri, 20 Aug 2010 08:35:02 -0700 (PDT)

```Many thanks David.
This was really helpful.
Nick

--- On Fri, 8/20/10, Airey, David C <david.airey@Vanderbilt.Edu> wrote:

> From: Airey, David C <david.airey@Vanderbilt.Edu>
> Subject: Re: st: power calculation graphs
> To: "statalist@hsphsun2.harvard.edu" <statalist@hsphsun2.harvard.edu>
> Date: Friday, August 20, 2010, 5:48 PM
> .
>
> While you can use -postfile-, you can also work directly
> with a Stata data file. Here's a simple example of making a
> power graph:
>
>
> clear
>
> set obs 7 // make a data set of 7 observations
>
> generate size = . // prepare variable for data
>
> generate power = . // prepare variable for data
>
> local i 1 // i is used to identify data rows
>
> foreach power of numlist 0.50(0.10)0.90 0.99 { // set up a
> loop
>
>     sampsi 9 13, sd1(2) sd2(4)
> power(`power') alpha(0.05)
>
>     replace size = r(N_1) in `i' // add size
> result to data set
>
>     replace power = `power'*100 in `i' //
> add power result to data set
>
>     local i = `i' + 1 // increment to the
> next row
>
> } // end the loop
>
> graph twoway connected power size, ///
>     xlabel(0(5)25) mc(red) lc(red) // make a
> graph
>
> graph export plot1.pdf, replace // Apple OS X only
>
>
> Also, there are power(ful) articles in the Stata Journal:
>
> Generalized power calculations for generalized linear
> models and more
> R. Newson. 2004.
> Stata Journal Volume 4 Number 4.
>
> Power by simulation
> A. H. Feiveson. 2002.
> Stata Journal Volume 2 Number 2.
>
>
> On Fri, Aug 20, 2010 at 8:54 AM, Nikolaos Pandis <npandis@yahoo.com>
> wrote:
> > Hi to all.
> >
> > I was wondering if someone would be able to help me
> draw the following sample calculation graphs with Stata:
> >
> > 1. Two arm parallel trial, outcome binary.
> >
> > A power (y-axis) sample size (x-axis) calculation
> graph for various failure proportions between trial arms.
> For example, p1=0.80 and p2=0.85, or p1=0.80 p=0.90 or
> p1=0.80 p=0.95
> >
> >
> > 2. Two arm parallel trial, outcome continuous.
> >
> > A power (y-axis) sample (x-axis) calculation graph for
> various failure proportions between trial arms. For example,
> m1= 100 and m2= 120 with equal sd1=sd2=10. It would be great
> if I could vary m1, m2, sd (always sd1=sd2)
> >
>
> You can do both of these using the -sampsi- command and
> could have
> found this yourself by typing -search sample size- into
> Stata.
>
> If you want to do the analysis for a range of values then
> you should
> consider putting each call within a series of nested
> -forval- loops
> and write the results to a file using -postfile- which you
> can then
> read into Stata and draw the graphs you want.
>
> I'll leave the actual coding of this as an exercise for you
> to work on ;-)
>
> Neil
>
>
>
> *
> *   For searches and help try:
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> *   http://www.stata.com/support/statalist/faq
> *   http://www.ats.ucla.edu/stat/stata/
>

*
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```